This invention relates to a method for improving the stability of residual fuel oils. More particularly, this invention relates to an improved economical method of inhibiting or reducing the level of asphaltic sediment separating from residual fuel oils by adding an effective stabilizing amount of an alkylaryl sulfonic acid stabilizer to the residual fuel oil and maintaining said stabilized fuel at a selected high temperature for a sufficient time to inhibit the formation or allow such reduction in sediment.
It is known that various types of instability may be exhibited by residual fuel oils. Among these are (1) separation of asphaltic or carbonaceous matter, sludge, dirt and water during storage at normal temperatures; (2) separation of black waxy material during storage at low temperatures; (3) increase in viscosity during storage at normal temperatures; and (4) incompatability or separation of insoluble matter on mixing of fuel oils from different sources.
Most present residual and intermediate fuel oils contain heavy asphaltic stocks in widely varying proportions. There is some evidence that certain constituents of these asphaltic stocks such as asphaltenes, carbenes and the like are colloidal in nature and thus blends containing such stocks would not be expected to form true solutions in all cases. Rather, some constituents would be dispersed in the blend and might separate under certain conditions of storage and use.
In the past, the precipitation of asphaltenes and resins from residual, i.e., residuum containing, fuels have been largely avoided by proper selection of blending components. Only distillate and residuum from the same or similar crudes were mixed so there was less likelihood of colloid destruction through changes in solvency. In addition, the severity of reduced crude processing was controlled to a level that produced distillate and residuum which, on reblending, provided compatible fuels. However, as crude availability tightened due to depletion of reserves and changes in political climate, and also as the need increased to process certain component fractions more severely to reduce sulfur levels, refining flexibility was lost. It became increasingly difficult to make components that would insure compatible blends, particularly those also meeting low sulfur specifications.
Additives of the detergent or dispersant type that are added to hydrocarbon fuels to control sludge separation are sometimes claimed to stabilize fuels against asphaltic constituent separation. However, most of them are either ineffective or only marginally effective at practical treating levels, especially for low sulfur intermediate fuels. Structurally, these additives are usually metal salts of alkylaryl sulfonic acids (see U.S. Pat. No. 2,888,338) or complex ashless dispersants containing amine, imide, ester or hydroxyl type polar functionality attached to an oil soluble hydrocarbon chain (see U.S. Pat. No. 2,958,590 and Canadian Pat. No. 605,449).
Oil soluble sulfonate additives have been taught to be useful for stabilization against oxidative deterioration (not sedimentation of asphaltic constituents) of middle distillate petroleum fuel oil compositions (see U.S. Pat. No. 2,923,611 and Canadian Pat. No. 607,389).
More recently in U.S. Pat. No. 4,182,613 the method of reducing the amount of asphaltic sediments in residual fuels through the use of alkylaryl sulfonic acids was disclosed. Generally while this technique is effective, it has been found that fairly high amounts of the sulfonic acid stabilizer and longer treatment times are needed to reduce the sediment to desired levels, making this method economically unattractive.